Abstract

Coarse-grained coffinite [USiO4•nH2O; n = 0–2] from the Upper Jurassic Morrison Formation in the Grants uranium region, New Mexico, USA, has been investigated in order to understand the processes of coffinite formation and alteration under reducing and oxidizing conditions. Elongated (up to 35 μm) prismatic crystals of coffinite precipitated contemporaneously with layers of organic matter (OM) and a vanadium-rich mica in this sandstone. Three different populations of coffinite were determined based on paragenesis and chemical composition; i) primary coffinite, (U0.924Ca0.218Y,REE0.021)1.163(Si0.835P0.033As0.010)0.878O4•nH2O, that was partially dissolved by migrating organic acids under reducing conditions; ii) recrystallized coffinite, (U0.805Ca0.156Y,REE0.022Zr0.001)0.984(Si0.954P0.036As0.005)0.995O4•nH2O, for which there was a 13% loss of U [apfu] and 28% loss of Ca [apfu], while Y + La + Ce + Nd [apfu] is enriched 5%. Uranium and REEs were incorporated into nano-scale crystals of secondary coffinite within the layers of organic matter; iii) coffinite formed during extensive alteration under oxidizing conditions. This coffinite, (U0.619Ca0.117Y,REE0.018)0.754(Si1.127P0.040As0.004)1.171O4•nH2O, is depleted ~ 23% of U [apfu], 25% of Ca [apfu] and 18% of Y +REE [apfu], as compared with coffinite-(ii). (Na,K)-boltwoodite, [(Na,K)(UO2SiO3OH)(H2O)], and a mixture of various uranyl sulfates, precipitated at the expense of coarse-grained coffinite-(i) as the final products of the alteration under oxidizing conditions.

Based on charge balance calculations of coffinite groups (i) and (ii), the amount of U6+ was estimated to be in the range of 0.1–0.19 [apfu]. An upper limit of 0.2 [apfu] U6+ is postulated for the coffinite structure. Thus, the ideal chemical formula of coffinite is: (U4+1 − xU6+x)Si1 − xO4•nH2O, where 0 < x < 0.2; 0 < n < 2. Compositional variations of coffinite are governed by the substitutions: U4+ + Si4+ ⇔ U6+ + [2(OH)−, 0.5 □]; REE3+ + As5+ ⇔ U4+ + Si4+; Ca2+ + (As,P)5+ ⇔ REE3+ + Si4+. The variable, but high, totals of the chemical analyses (92.8–99.7 wt.%) suggest that coffinite contains molecular H2O, which is not an essential component of coffinite structure. The amount of H2O varies between 0 and 2 molecules per formula unit, and a minor amount of (OH)− may be accommodated into the structure.

The U–Pb chemical ages limit the age of coffinite precipitation to between 36.6 and 0.7 Ma, which indicates continuous precipitation of coffinite since the mid-Tertiary. These results suggest that organic matter, which has preserved reducing conditions even in the presence of oxidizing fluids, over this long period of approximately 30 million years, plays an important role in the sustained presence of coffinite in these deposits.